This invention relates to an amplitude equalizing circuit and more particularly a variable resonance type amplitude equalizing circuit for equalizing the amplitude distortion caused by fading in a wireless communication system, for example.
In a wireless communication system, in addition to an electric wave sent directly from a transmission station, waves reflected from the ground, mountains, sea and buildings or waves which are caused to refract many times by ducts are generally received so that so-called selective or multipath fading occurs. When a modulated wave is subjected to such fading, the effect of fading is caused to vary due to a small difference in the side band frequency which causes an amplitude variation and hence a transmission distortion. Various amplitude equalizing circuits have been proposed for the purpose of preventing such fading.
Generally such amplitude equalizing circuit comprises first and second input terminals, first and second output terminals, a variable resistor connected between the first input terminal and the first output terminal for adjusting the peak of an equalizing characteristic curve, and a line having a wavelength 1/4 of the center frequency of the fading and connected between the first and second output terminals. Generally, the second input terminal and the second output terminal are connected in common and maintained at the ground potential. However, such amplitude equalizing circuit is not advantageous in that it is necessary to vary the length of the line as the center frequency of the fading varies. For the purpose of equivalently varying the length of the line we have already proposed to connect a series resonance circuit comprised by a coil and a capacitor in series with the line as disclosed in Kozo Morita's paper "Variable Resonance Type Automatic Equalizer" Institute of Electronic Communication (Japan), 61th Anniversary General Meeting Papers Vol. 8, 8-92. With the circuit disclosed in this paper, in order to increase the degree of variation in the line length it is necessary to increase the variation in the resonance frequency of the series resonance circuit, that is the degree of variation in the inductance of the coil and the electrostatic capacitance of the capacitor. However, it is impossible to obtain an inductive element capable of operating at frequencies above a VHF band and which can effectively vary its inductance. Accordingly, the width of variation in the resonance frequency is solely determined by the range of variation in the electrostatic capacitance of the capacitor. For this reason it is impossible to equivalently vary the line length in a large amount. With the above described circuit utilizing a series resonance circuit, in a frequency band of 100-200 MHz, it was possible to vary the bandwidth of the center frequency by only .+-.15 MHz. In other words, it has been impossible to equalize the amplitude distortion of the center frequency of the fading over a wideband.